[10] Early demonstrations included Pneumatic Diversity Vents, shown to transport objects and transfer suction power through portals, but these do not appear in the final game.

[22] Before Portal, Aperture Science conducted experiments to determine whether human subjects could safely navigate dangerous "test chambers", until the artificial intelligence GLaDOS, governing the laboratory, killed its employees.

At the end of the first Portal, the protagonist Chell destroys GLaDOS and momentarily escapes the facility, but is dragged back inside by an unseen figure later identified by writer Erik Wolpaw as the "Party Escort Bot".

Wheatley (Stephen Merchant), a personality core, guides her through old test chambers from the first game in an attempt to escape the facility after its reserve power is depleted.

[24][25] They accidentally reactivate the dormant GLaDOS (Ellen McLain)[2] while attempting to restore power to the escape pods; she separates Chell from Wheatley and begins rebuilding the facility.

[30] His last request was for the mind of his assistant Caroline (McLain) to be transferred—by force, if necessary—to an advanced computer designed to store a human consciousness, which he had previously commissioned to save himself, creating GLaDOS.

[43] Project manager Erik Johnson said Valve's goal for Portal 2 was to find a way to "re-surprise" players, which he considered a "pretty terrifying" prospect.

[49][50] Valve's F-Stop game was set in the 1980s, and would not have featured Chell or GLaDOS; instead, it followed a new test subject involved in a conflict within Aperture after Johnson, in an attempt to reach immortality, uploaded himself into an artificial intelligence and took control of a robot army.

[66] The writers felt they needed to create a larger story for a stand-alone title, and wanted the game to "feel relatively intimate", and avoided adding too many new characters.

[11][47] In June 2008, based on information from a casting call website and leaked script samples, Kotaku reported that Valve was seeking voice actors to play Johnson, named him as an AI and identified the game as a prequel.

[68] The cooperative campaign was planned to feature a more detailed storyline, in which GLaDOS would send two robots to discover human artifacts, such as a comic based on a pastiche of Garfield.

As an easter egg, a hidden area in Portal 2 contains the empty dry dock of Aperture Science's cargo ship, the Borealis, which is found during Half-Life 2: Episode Two to have been stranded in the Arctic as a result of a teleportation experiment.

[85] As part of her character arc, the plot moves GLaDOS from her anger with Chell for her actions in Portal, which Wolpaw said "was going to get old pretty quick", to an internal struggle.

[96] Portal 2 contains both scored and procedurally generated music created by Valve's composer, Mike Morasky,[59][97] and two songs; "Want You Gone" recorded by Jonathan Coulton, used on the final credits of the single-player mode,[98] and "Exile Vilify" by the National, used in the background of one of the Rat Man's dens.

[131] An earlier video released on February 14, 2011, promoted the cooperative aspect of Portal 2 as a St. Valentine's gift and "lit up our preorders, our buzz, all the metrics that are used and collected by publishers and retailers".

[145] The announcement that Portal 2 would be available on PlayStation 3 came as a surprise to the industry because Gabe Newell had criticized that console in the past, citing difficulties in the port of The Orange Box.

[155] Since February 2014, SteamOS, Valve's own Linux-based operating system, supports Portal 2 and its predecessor, as do most modern Debian-based Linux distributions on its services via the Steam client.

Released as a Beta in early 2014 for Linux distributions, it holds all of the same traits as the other versions, retaining cross-platform play, split screen and fully native controller support.

[156] A feature called "Robot Enrichment" allows players to customize the cooperative campaign characters with new gestures and cosmetic items such as hats or flags.

[160] The content also adds a "challenge mode" similar to that in Portal—players try to complete specific chambers with the shortest time or fewest portals used, both which are tracked on overall and friends leaderboards.

[164] Wolpaw and McLain also helped to create additional lines for GLaDOS for a custom single-player map commissioned by Gary Hudston, which he used to propose marriage to his fiancée, Stephanie.

[165][166] For a patch for Bethesda's The Elder Scrolls V: Skyrim that incorporated support for Steam Workshop content, Valve developed a free add-on module that introduced the Space Core as a non-player character that follows the player around.

[168] A Portal 2-themed set is available for Lego Dimensions by Warner Bros. Entertainment and Traveller's Tales; the game features additional stories written by Traveller's Tales with Valve's blessing set after the events of Portal 2, with Ellen McLain, Stephen Merchant and J.K. Simmons reprising their respective voice roles, as well as a new GLaDOS credits song written by Jonathan Coulton and performed by McLain.

[196][199][212][220] Onyett wrote that Merchant's "obvious enthusiasm for the role benefits the game" and that the "consistently clever writing perfectly complements the onscreen action".

[200] In contrast, Peter Bright of Ars Technica wrote that compared to the loneliness and despair he felt while playing the first game, the characters, Wheatley and GLaDOS, lost some of this feeling and "the inane babble served only to disrupt the mood".

[14][222][198][201][212][214][223] Time's Evan Narcisse said that he feared the addition of new gameplay elements would "dilute the purity of the experience, but everything's still executed with Valve's high level of charm and panache.

"[219] Tom Hoggins of The Telegraph praised the manner with which these elements were introduced through a "brilliant learning curve of direction, rather than instruction", and considered it a "design ethos that is supremely generous, but dealt with marvellous economy".

[202] Hoggins wrote that the game's world reacted to the player-character Chell's presence "in a startlingly organic way", and praised Valve's design as "an achievement of world-building that compares favourably with BioShock's underwater city of Rapture".

Young wrote that in the second act, the game "cranks up the difficulty level at a speed that may dishearten casual gamers", and said that particularly when traveling between chambers, he had "absolutely no idea where I was supposed to head next".

"[222] Mathematics and science teachers wrote e-mails to Valve to tell them how they had included Portal in their classroom lessons as part of a project to promote the "gamification of learning".